In the context of an internal combustion engine, it is known to deliver fuel into the cylinders of the engine by means of a fuel injector. One such type of fuel injector that permits precise metering of fuel delivery is a so-called ‘piezoelectric injector’.
With reference to FIG. 1, a piezoelectric injector 2 includes a piezoelectric actuator 4 that is operable to control the position of an injector valve needle 6 relative to a valve needle seat 8. Depending on a drive voltage profile ‘V’ applied to the piezoelectric actuator 4, the valve needle 6 is either caused to disengage the valve seat 8, in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 10, or is caused to engage the valve seat 8, in which case fuel delivery is prevented.
The piezoelectric injector is controlled by an injector control unit (ICU) 20 that forms an integral part of an engine control unit (ECU) 22. The ECU continuously monitors a plurality of engine parameters 24 and feeds an engine power requirement signal to the ICU 20. The ICU 20 calculates (using processor 21) a required injection event sequence to provide the required power for the engine and outputs a voltage pulse profile 25 to an injector drive circuit 26. In turn, the injector drive circuit 26 applies the voltage drive profile 25 to the injector via a high side voltage signal VHI and a low side voltage signal VLO.
In order to initiate an injection, the drive circuit 26 causes the differential voltage between VHI and VLO to transition from a high voltage (typically 250V) at which no fuel delivery occurs, to a relatively low voltage (typically 50 V), which initiates fuel delivery. An injector responsive to this drive waveform is referred to as a ‘de-energise to inject’ injector.
Such a fuel injector is operable to deliver one or more injections of fuel within a single injection event. For example, the injection event may include one or more so-called ‘pre’ or ‘pilot’ injections, a main injection, and one or more ‘post’ injections. In general, several such injections within a single injection event are preferred to increase combustion efficiency of the engine.
A typical injector drive voltage profile applied to the injector during an injection event is shown in FIG. 2 and a corresponding ideal delivery rate profile is shown in FIG. 3.
The injector drive voltage profile comprises first and second pilot discharge pulses P1 and P2 and a single main injection discharge pulse PMAIN. The magnitude and duration of each of the pilot discharge pulse P1, P2 are substantially equal. Accordingly, the delivery rate for each pilot injection P1, P2 is substantially equal and, thus, the volume of fuel delivered (the area under the curve) is consistent between pilot injections.
It has been observed, however, that the actual delivery quantity between pilot injections for the same voltage discharge profile varies considerably. For example, FIG. 4 shows a delivery rate profile that is observed in practice in which the fuel delivered for the second pilot injection is greater than the fuel delivered during the first pilot injection.
The purpose of a pilot injection is to deliver a precise amount of fuel into the combustion chamber prior to the main injection in order to initiate the combustion process gradually. Therefore, a variation in fuel delivery between pilot injections is undesirable since it reduces the controllability of the combustion process. Therefore, a method of regulating the volume of fuel delivered between pilot injections is required.